This invention relates to an exposure method, an exposure apparatus and a device manufacturing method. In one preferred embodiment, the invention is concerned with a dual or multiple exposure method for printing a fine circuit pattern on a photosensitive substrate, and an exposure apparatus and a device manufacturing method based on the dual (multiple) exposure method. The exposure method and apparatus of the present invention are suitably usable in the manufacture of microdevices such as semiconductor chips (e.g., IC or LSI), display devices (e.g., a liquid crystal panel), detecting devices (e.g., a magnetic head), or image pickup devices (e.g., a CCD), for example.
In the manufacture of devices such as IC, LSI, or liquid crystal panels, for example, on the basis of photolithography, generally, a projection exposure method and a projection exposure apparatus are used, in which a circuit pattern of a photomask or reticle (hereinafter xe2x80x9cmaskxe2x80x9d) is projected by a projection optical system onto a photosensitive substrate (hereinafter xe2x80x9cwaferxe2x80x9d) such as a silicon wafer or glass plate having a photoresist coating applied thereto, for example, whereby the pattern is transferred or printed on the substrate.
In order to meet further increases in the density of integration of such devices, further miniaturization of a pattern to be transferred to a wafer (that is, further improvement of resolution) as well as a further increase of the area of a single chip are required. In projection exposure methods and projection exposure apparatuses which are the primary types of microfabrication technology, attempts have been made to improve the resolution and the exposure area that an image of a size (linewidth) of 0.5 micron or less can be formed in a wide range.
FIG. 19 is a schematic view of a projection exposure apparatus of a known type. Denoted in FIG. 19 at 191 is an excimer laser which is a light source for deep ultraviolet light exposure. Denoted at 192 is an illumination optical system, and denoted at 193 is illumination light. Denoted at 194 is a mask, and denoted at 195 is object-side exposure light which, after being emitted from the mask 194, enters an optical system 196. The optical system 196 comprises a reduction projection optical system. Denoted at 197 is image-side exposure light which, after being emitted from the optical system 196, impinges on a photosensitive substrate 198. The substrate 198 comprises a wafer. Denoted at 199 is a substrate stage for holding the photosensitive substrate 198.
Laser light emitted by the excimer laser 191 is directed by a guiding (or directing) optical system to the illumination optical system 192. By means of the projection optical system 192, the light is adjusted or transformed into illumination light 193 having a predetermined light intensity distribution, an orientation distribution, and an opening angle (numerical aperture NA), for example. The illumination light 193 illuminates the mask 194. The mask 194 has a fine pattern of chromium, for example, formed on a quartz substrate. The pattern has a size corresponding to an inverse (e.g., 2xc3x97, 4xc3x97, or 5xc3x97) of the projection magnification of the projection optical system 192. The illumination light 193 is transmissively diffracted by the fine pattern of the mask 194, whereby object-side exposure light 195 is provided. The projection optical system 196 serves to transform the object-side exposure light 195 into image-side exposure light 197 with which the fine pattern of the mask 194 is imaged upon the wafer 198, at the above-described projection magnification and with sufficiently small aberration. As illustrated in an enlarged view at the bottom of FIG. 19, the image-side exposure light 197 is converged upon the wafer 198 with a predetermined numerical aperture (NA=sin xcex8), whereby an image of the fine pattern is formed on the wafer 198. For sequentially printing the fine pattern on different regions (shot regions, each being a region for the production of one or plural chips), the substrate stage 199 moves stepwise along an image plane of the projection optical system to change the position of the wafer 198 with respect to the projection optical system 196.
Practically, however, with current projection exposure apparatuses having an excimer laser as a light source, it is difficult to form a pattern of 0.15 micron or less.
In the projection optical system 196, there is a limitation in resolution due to a tradeoff between optical resolution and depth of focus which is attributable to the wavelength of exposure light (hereinafter xe2x80x9cexposure wavelengthxe2x80x9d). The relation between resolution R and depth of focus DOF of a projection exposure apparatus can be expressed in accordance with Rayleigh""s equation, such as equations (1) and (2) below:
xe2x80x83R=k1(xcex/NA)xe2x80x83xe2x80x83(1)
DOF=k2(xcex/NA2)xe2x80x83xe2x80x83(2)
where xcex is the exposure wavelength, NA is the image-side numerical aperture that represents brightness of the projection optical system 196, and k1 and k2 are constants which are usually about 0.5-0.7. It is seen from equations (1) and (2) that, in order to provide higher resolution with a smaller resolution value R, the numerical aperture NA may be enlarged (enlargement of NA). However, in a practical exposure process, the depth of focus DOF of the projection optical system 196 should be not less than a certain value and, therefore, enlargement of the numerical aperture NA beyond a certain level is not practicable. Thus, for higher resolution, it is necessary to make the exposure wavelength xcex shorter (shortening of wavelength).
However, shortening of the wavelength raises a serious problem. That is, there is no lens glass material available for the projection optical system 196. Almost all glass materials have about a zero transmissivity to the deep ultraviolet region. While there is a fused silica which can be produced as a glass material in an exposure apparatus with an exposure wavelength about 248 nm in accordance with a special manufacturing method, the transmissivity of even such fused silica decreases drastically to an exposure wavelength not longer than 193 nm. Thus, it may be very difficult to develop a practical glass material having a sufficiently high transmission factor in a region not longer than an exposure wavelength of 150 nm, corresponding to a fine pattern of 0.15 micron or less. Further, the glass material to be used in the deep ultraviolet region should, to some extent, satisfy several other conditions such as durability, uniformness of birefringence or refraction factor, optical distortion, and workability or machining characteristic, for example. For these reasons, development of a practical glass material for use in an exposure wavelength region not longer than 150 nm will not easily be accomplished.
In conventional projection exposure methods and projection exposure apparatuses, such as described, for the formation of a pattern of 0.15 micron or less upon a wafer 198, the exposure wavelength should be shortened to about 150 nm or less. Nevertheless, since there is no practical glass material available for such a wavelength region, practically, it is very difficult to form a pattern of 0.15 micron or less on the wafer 198.
U.S. Pat. No. 5,415,835 shows a process of forming a fine pattern by use of dual-beam interference exposure (also known as xe2x80x9cdouble-beam interference exposurexe2x80x9d). This exposure process involves the use of two mutually coherent light beams that interfere with each other to produce an interference fringe. With this dual-beam interference exposure process, a pattern of 0.15 micron or less may be formed on a wafer.
Referring to FIG. 15, the principle of dual-beam interference exposure will be explained. In accordance with dual-beam interference exposure, laser light from a laser 151 which comprises parallel light having coherency is divided by a half mirror 152 into two light beams. These light beams are then reflected by flat mirrors 153, such that the two laser light beams (coherent parallel light beams) intersect with each other at an angle not less than zero deg. and not greater than 90 deg., whereby an interference fringe is produced at the intersection. A wafer 154 is exposed and sensitized by use of this interference fringe (i.e., the light intensity distribution of it), by which a fine periodic pattern corresponding to the intensity distribution of the interference fringe is formed on the wafer.
When the two light beams intersect at the wafer surface in a state wherein they are inclined with respect to a normal to the wafer surface oppositely by the same angle, the resolution R attainable with this dual-beam interference exposure process can be expressed by equation (3) below:
xe2x80x83R=xcex/(4 sin xcex8)
=xcex/4NA
=0.25(xcex/NA)xe2x80x83xe2x80x83(3)
where R represents widths of line and space, respectively, that is, widths of bright and dark portions of the interference fringe, respectively, and xcex8 denotes an incidence angle (absolute value) of the two light beams with respect to the image plane. (AS noted above, NA=sin xcex8.)
Comparing equation (1) for resolution according to an ordinary projection exposure process with equation (3) for resolution according to a dual-beam interference exposure process, since resolution R in the dual-beam interference exposure corresponds to that in a case where k1=0.25 in equation (1), it is seen that with the dual-beam interference exposure, a resolution two or more times higher than that of an ordinary projection exposure process (k1=0.5 to 0.7) can be provided. Although it is not discussed in the aforementioned U.S. patent, if xcex=0.248 nm (KrF excimer laser) and NA=0.6, a resolution R=0.10 micron may be attainable.
In accordance with the dual-beam interference exposure process just described, however, basically only a simple fringe pattern corresponding to the light intensity distribution of an interference fringe (i.e., exposure amount distribution) is attainable. It is not possible to produce a complicated pattern of a desired shape, such as a circuit pattern, on a wafer using this exposure process.
The aforementioned U.S. Pat. No. 5,415,835 proposes a procedure in which, after a simple (periodic) exposure amount distribution is applied to a resist of a wafer through an interference fringe by using a dual-beam interference exposure apparatus, a separate exposure apparatus is used so that a portion of the resist corresponding to the bright portion of the interference fringe is exposed to an image of an opening of a mask by which a certain exposure amount is applied to that portion (dual exposure). By this, the exposure amounts only at particular line portions of plural bright portions of the interference fringe are enlarged uniformly, beyond the threshold of the resist. Consequently, after development, isolated lines (resist pattern) are produced.
With this dual exposure method proposed in U.S. Pat. No. 5,415,835, however, what is attainable is only a circuit pattern of a simple shape which comprises a portion of stripe patterns that can be formed by double-beam interference exposure. On the other hand, an ordinary circuit pattern comprises a combination of many types of patterns having various linewidths and various orientations. It is, therefore, not attainable to produce a complicated pattern such as a circuit pattern.
Further, while the aforementioned U.S. Pat. No. 5,415,835 discusses a combined use of a dual-beam interference exposure process and an ordinary exposure process, it does not mention the structure of an exposure apparatus suitable for this combination.
Japanese Laid-Open Patent Application, Laid-Open No. 253649/1995 shows a dual exposure method with which a fine isolated pattern similar to that of the aforementioned U.S. Pat. No. 5,415,835 may be formed. In accordance with this dual exposure method, an ordinary projection exposure apparatus is used to perform both a double-beam interference exposure based on a phase shift pattern and an exposure based on an image of a fine opening pattern (which are not resolvable with this exposure apparatus), to the same region on a resist of a wafer. The exposure wavelengths used in these exposures differ from each other by 50 nm or more.
Further, in the dual exposure method shown in Japanese Laid-Open Patent Application, Laid-Open No. 253649/1995, the pattern of the mask is formed by use of a material having a wavelength selectivity such that the double-beam interference exposure and the ordinary exposure are performed by using one and the same mask (pattern). The pattern (of the mask) in the ordinary exposure comprises one or more isolated patterns, and also, the circuit pattern (exposure amount distribution or surface step distribution after development) produced as a result of dual exposure comprises one or more isolated patterns, only.
Therefore, even with the dual exposure method shown in Japanese Laid-Open Patent Application, Laid Open No. 253649/1995, like the aforementioned U.S. Pat. No. 5,415,835, it is not attainable to produce a pattern of a complicated shape such as a circuit pattern.
It is an object of the present invention to provide an exposure method and/or an exposure apparatus by which a circuit pattern of a complicated shape can be produced through multiple exposure. Here, the words xe2x80x9cmultiple exposurexe2x80x9d refer to a process wherein exposures are made to the same location on a resist without intervention of a development process between these exposures. The exposures may be double or triple or more.
It is another object of the present invention to provide a device manufacturing method and/or an exposure apparatus that uses a dual or multiple exposure method.
In accordance with an aspect of the present invention, there is provided an exposure method for dual or multiple exposure, comprising the steps of:
performing a first exposure process by use of an interference fringe produced by interference of two light beams; and performing a second exposure process by use of a light pattern different from the interference fringe; wherein, in at least one of the first and second exposure processes, a multiplex exposure amount distribution is provided.
In one preferred form of this aspect of the present invention, in the second exposure process, a multiplex exposure amount distribution may be applied.
In one preferred form of this aspect of the present invention, the second exposure process may be performed by use of plural masks having different patterns.
In one preferred form of this aspect of the present invention, the second exposure process may be performed by use of a mask with plural transparent regions having different transmissivities.
In one preferred form of this aspect of the present invention, the first exposure process may be performed by use of a pattern of a phase shift mask and a projection exposure apparatus.
In one preferred form of this aspect of the present invention, the first exposure process may be performed by use of an interferometer.
In one preferred form of this aspect of the present invention, the first and second exposure processes may be performed by use of a projection exposure apparatus.
In one preferred form of this aspect of the present invention, the first exposure process may be performed by use of a phase shift mask.
In one preferred form of this aspect of the present invention, in the first exposure process, a multiplex exposure amount distribution may be applied.
In accordance with another aspect of the present invention, there is provided a device manufacturing method including a step for transferring a device pattern onto a workpiece by use of an exposure method as recited above.
In accordance with a further aspect of the present invention, there is provided a projection exposure apparatus for performing an exposure method as recited above.
The first and second exposure processes may be performed sequentially or at the same time. When they are performed sequentially, basically, either may be performed first.
Here, the word xe2x80x9cmultiplexxe2x80x9d referred to above in relation to the phrase xe2x80x9cmultiplex exposure amount distributionxe2x80x9d means that, unlike a binary exposure amount (two levels including a zero level exposure amount) to be applied to a photosensitive substrate, more than a binary exposure amount (three or more levels including a zero level exposure amount) is given. Further, the words xe2x80x9cordinary exposure (process)xe2x80x9d are used to refer to an exposure process which is to be done with a resolution lower than that attainable with dual-beam interference exposure and to be done with a pattern different from that used in the dual-beam interference exposure. A typical example of such an ordinary exposure process is projection exposure for projecting a mask pattern through a projection exposure apparatus such as shown in FIG. 19.
Each of the dual-beam interference exposure process and ordinary exposure process to be performed in the present invention may comprise a single exposure step or plural exposure steps. In the latter case, in each step, a different exposure amount may be applied to a photosensitive substrate.
In an exposure method and exposure apparatus according to the present invention, if the second exposure process is to be performed through projection exposure, the first and second exposure processes may use exposure wavelengths not greater than 400 nm, preferably not greater than 250 nm. Exposure light of a wavelength not greater than 250 mm may be available from a KrF excimer laser (about 248 nm) or an ArF excimer laser (about 193 nm).
An exposure apparatus according to the present invention may comprise a projection optical system for projecting a pattern of a mask onto a wafer, and a mask illumination system for selectively providing partially coherent illumination and coherent illumination. An ordinary exposure process may be performed with partially coherent illumination, while dual-beam interference illumination may be performed with coherent illumination. Here, the words xe2x80x9cpartially coherent illuminationxe2x80x9d are used to refer to an illumination mode with "sgr" (=xe2x80x9cnumerical aperture of the illumination optical systemxe2x80x9d divided by xe2x80x9cnumerical aperture of the projection optical systemxe2x80x9d) which is larger than zero and smaller than one. The words xe2x80x9ccoherent illuminationxe2x80x9d are used to refer to an illumination mode with "sgr" which is equal to or close to zero, it being very small as compared with the a value of the partially coherent illumination.
The exposure apparatus described just above may use an exposure wavelength not greater than 400 nm, preferably not greater than 250 nm. Exposure light of a wavelength not greater than 250 mm may be available from a KrF excimer laser (about 248 nm) or an ArF excimer laser (about 193 nm).
One preferred embodiment of the present invention, to be described later, includes an optical system for a mask illumination optical system that enables interchanging between partially coherent illumination and coherent illumination.
An exposure system according to another preferred embodiment of the present invention may comprise a combination of a dual-beam interference exposure apparatus and an ordinary (projection) exposure apparatus, and a movement stage for holding a workpiece (photosensitive substrate) and being used in both of these apparatuses. This exposure system may use an exposure wavelength not greater than 400 nm, preferably not greater than 250 nm. Exposure light of a wavelength not greater than 250 mm may be available from a KrF excimer laser (about 248 nm) or an ArF excimer laser (about 193 nm).
In accordance with another aspect of the present invention, there is provided an exposure method and exposure apparatus for exposing a resist with a mask having pattern portions being different with respect to contrast of image, wherein the position where an image of a pattern portion, of the mask, having lowest contrast of image is formed is exposed with an image of contrast higher than the lowest contrast image, whereby contrast of an exposure amount distribution related to the pattern portion of lowest contrast is improved.
In accordance with still another aspect of the present invention, there is provided an exposure method and apparatus for exposing a resist with a mask having pattern portions being different with respect to linewidth, wherein the position where an image of a pattern portion, of the mask, having a smallest linewidth is formed is exposed with an image of contrast higher than the image of the smallest linewidth pattern portion, whereby contrast of an exposure amount distribution related to the pattern of lower contrast is improved.
In accordance with a further aspect of the present invention, there is provided an exposure method and exposure apparatus for exposing a resist with a mask having plural pattern portions being different with respect to contrast of image, wherein multiple exposure to be performed with the method or apparatus includes a first exposure in which an exposure amount by an image of a pattern portion, of the pattern portions of the mask, of lowest contrast does not exceed an exposure threshold of the resist while an exposure amount by an image of another pattern portion exceeds the exposure threshold, and a second exposure in which the position where the image of lowest contrast is formed is exposed with an image of contrast higher than the image of lowest contrast.
In accordance with a still further aspect of the present invention, there is provided an exposure method and exposure apparatus for exposing a resist with a mask having plural pattern portions being different with respect to linewidth, wherein multiple exposure to be performed with the method or apparatus includes a first exposure in which an exposure amount by an image of a pattern portion, of the pattern portions of the mask, of smallest linewidth does not exceed an exposure threshold of the resist while an exposure amount by an image of another pattern portion exceeds the exposure threshold, and a second exposure in which the position where the image of smallest linewidth is formed is exposed with an image of a contrast higher than the image of lowest contrast.
In one preferred form of these aspects of the present invention, the resist may be exposed with images of patterns of the mask by use of radiation such as ultraviolet rays, X-rays, or an electron beam, for example, and with the use of or without use of a projection optical system.
The image of high or higher contrast may be formed by use of radiation of the same wavelength as the aforementioned radiation.
The resist may be exposed with the higher contrast image and the image of the pattern simultaneously. The resist may be exposed with the higher contrast image and, thereafter, it may be imaged with the image of the pattern. The resist may be exposed with the image of the pattern and, thereafter, it may be exposed with the higher contrast image.
The image of higher contrast may be formed by projecting a mask of a phase shift type. The phase shift type mask may comprise a Levenson-type phase shift mask. The phase shift type mask may include a phase shifter portion for applying a mutual phase shift of 180 deg. to radiation beams passing through two regions, respectively, without passing a light blocking portion. The phase shift type mask may include an isolated pattern provided by the phase shifter portion. The phase shift type mask may include a repetition pattern having arrayed phase shifter portions.
The image of higher contrast may be formed by projecting two parallel lights, resulting from division of laser light, onto the resist in different directions, to cause interference of them on the resist. The image of higher contrast may be formed by using a probe of light or electrons. The image of higher contrast may be formed by illuminating a repetition pattern of the mask along an oblique direction and by projecting it.
The first-mentioned mask may comprise a phase shift mask. The first-mentioned mask may comprise a phase shift mask of one of halftone type, rim type and chromium-less shifter light blocking type, with a result of a good-contrast exposure amount distribution.
The pattern of the first-mentioned mask may be illuminated along an oblique direction and may be projected by a projection optical system. The image of higher contrast may be formed in a state where a is not greater than 0.3 and by imaging a pattern of a phase shift mask, wherein the phase shift mask may comprise a Levenson-type phase shift mask.
In one preferred form of an exposure method and exposure apparatus of the present invention, the center position of the intensity distribution of an image of the pattern of the mask should be registered with the center position of the intensity distribution of the image of higher contrast. However, from the relation with contrast of an exposure amount distribution to be finally formed upon the resist, deviation within a certain range is allowed to the center positions of the intensity distributions of these images.
There is no limitation to exposure wavelength, in the present invention. However, the present invention is particularly suitably usable with an exposure wavelength of 250 nm or shorter. An exposure wavelength not longer than 250 nm may be provided by use of a KrF excimer laser (about 248 nm) or an ArF excimer laser (about 193 nm).
The present invention may be embodied, for example, by use of a projection exposure apparatus comprising a projection optical system for projecting a pattern of a mask to a wafer, and a mask illumination optical system which can perform (large "sgr") partial coherent illumination wherein "sgr" (sigma) is relatively large, and (small "sgr") partial coherent illumination wherein "sgr" is relatively small or coherent illumination. For example, projection exposure of the mask pattern (circuit pattern) may be performed through the large "sgr" partial coherent illumination, while a phase shift type mask may be illuminated through coherent illumination or small "sgr" partial coherent illumination. With such double-beam interference illumination, exposure of a higher contrast image by an interference image can be made.
The words xe2x80x9cpartial coherent illuminationxe2x80x9d refer to illumination wherein the value of "sgr" (=xe2x80x9cmask side numerical aperture of the illumination optical systemxe2x80x9d/xe2x80x9cmask side numerical aperture of the projection optical systemxe2x80x9d) is larger than zero and smaller than 1. The words xe2x80x9ccoherent illuminationxe2x80x9d refer to one in which the value of g is zero or close to zero, and it is very small as compared with "sgr" of partial illumination. A large "sgr" refers to "sgr" not smaller than 0.6, while a small "sgr" refers to "sgr" not larger than 0.3.
The exposure apparatus may include an optical system for the mask illumination optical system, wherein partial coherent illumination, coherent illumination, and partial coherent illumination of a relatively small "sgr" can be interchanged.
The present invention may be embodied by an exposure system which includes a double-beam interference exposure apparatus such as shown in FIG. 15, a projection exposure apparatus such as shown in FIG. 19, and a movement stage used in both of these exposure apparatuses for holding a wafer (photosensitive substrate). An exposure wavelength to be used may be not longer than 400 nm as described and, particularly, not longer than 250 nm. Light of an exposure wavelength not longer than 250 nm may be provided by use of a KrF excimer laser (about 248 nm) or an ArF excimer laser (about 193 nm).
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.